Nature shows various approaches to create superhydrophobicity, such as the lotus leaf, where the superhydrophobic (SHPB) surface arising from its hierarchical surface consists of random microscale bumps with superimposed nanoscale hairs. Some natural systems, such as the hydrophilic silk of some spider's webs, even allow the passive transport of water droplets from one part of a surface to another by creating gradients in surface tension and Laplace pressure. We look to combine both ideas and replicate the superb water repellence of the lotus leaf and the surface tension gradient-driven motion of the spider silk to form an all-metal, coating-less surface that promotes spontaneous droplet motion. We present the design, fabrication, and investigation of such superhydrophobic gradient surfaces on aluminum, which are aimed at spontaneous water droplet movement for improved surface water management. One surface demonstrates a droplet travel distance of almost 2 mm for a 11 μL droplet volume. We also present surfaces that map the theoretical ranges of the surface tension gradient surfaces tested here.

Superhydrophobic surfaces demonstrate extreme water-repellence, promoting drop-wise over film-wise condensation, increasing liquid mobility, and reducing thermal resistance for heat-exchanger applications. Introducing topographic structures can lead to modified surface free energy, as inspired by natural systems like the lotus leaf, potentially allowing coating-free ice- and frost-free surfaces under certain conditions. This work presents a study of coating-free aluminum micro/nanopatterns fabricated using micromilling or laser-etching techniques and the resultant wetting properties. Our review and experiments clarify the roles of line-edge-roughness and microstructural geometry from each microfabrication technique, which manifests in technique-specific nano- to midmicro-scale roughness, producing a hierarchical structure in both cases. For micromilling, line-edge-roughness consists of jagged burrs of 1–8 μm thickness with 10–25 μm periodicity along the microlines with constantly changing height on the order of 1–20 μm. These effects simultaneously raise the water contact angle from 52° (unprocessed aluminum) up to 136° but with strong edge pinning effects. On the other hand, laser-etched surfaces exhibit line-edge-roughness with a microstructure of 3–20 μm width and 5–10 μm in height superimposed with evenly spread spikes of 50–250 nm. This results in a high contact angle (>150°) coupled with a low contact angle hysteresis (<15°), promoting superhydrophobicity on a coating-free aluminum surface. It is also shown that for certain cases, line-edge-roughness is more important for the resultant wetting properties than the structure geometry.

Passive gradient-driven droplet motion has been demonstrated in nature, inspiring coating-free surface tension gradient surfaces that can be fabricated via laser ablation. These surfaces can potentially enhance heat exchanger performance, promoting drop-wise over film-wise condensation, and be suitable for lab-on-a-chip applications, allowing the directional transport of microliter size droplets. In this work, a theoretical model and its application to variable-pitch hierarchical superhydrophobic gradients are discussed, and the method is experimentally validated against various gradient topographical designs. The proposed force balance model allows analysis of the impact of the topography on the forces acting on the droplet. The discrepancy between modeled and observed contact angles in most cases does not exceed 10%. The modeled droplet footprint fits the experimentally measured ones with an error of less than 10% for most cases. Though modeled motion distances were twice greater than experimentally observed ones, the comparison of the proposed model with the originally developed theory showed that the difference in the net force was less than 5%. Both observed and average velocities were within less than 30% difference. Like the traditional models, the new model overestimates droplet kinematics; however, it does not require knowledge a priori of all the contact angles across the gradient during droplet motion, relying only on the material's surface tension and the local surface area fraction. Therefore, the model presents a simplified and convenient means of designing a linear topographical gradient for spontaneous droplet motion.